There has been known a digital radio communication system in which, when digital information is transmitted via a radio communication system, the information (such as a bit string of binary signals) is divided into a prespecified number of bits to generate a prespecified frame structure including the divided bit string, and a received base band signal is modulated to a digital signal by means of the modulation system such as the π/4 shift QPSK (Quadrature Phase Shift Keying) system to transmit the digital signal as a digitally modulated signal. The digital radio communication system as described above is widely used as an AVM (Automatic Vehicle Monitoring) system, for instance, for allocating taxis with a radio communication system. In this case a particular communication station functions as a master station (or a base station), and a number of communication terminal stations (which are sometimes referred to as in-vehicle stations or as slave stations) makes communications under control by the master station.
In the digital radio communication system as described above, it is necessary for the master station to always recognize information about a current state of each vehicle including a position of a slave station in the car, whether a passenger is in the vehicle or not, and whether any abnormality has occurred in the vehicle or not, and the communication system referred to as “polling system” is widely used. Refer to, for instance, Japanese Patent Laid-Open Publication No. 5-284561 (patent document 1).
The digital radio communication system as described above is described below with reference to FIG. 5. FIG. 5 schematically shows a case in which the digital communication system as described above is applied to a taxi radio communication system. For instance, in-vehicle stations 502-1, 502-2, . . . 502-N are connected to a master station 501. In the digital radio communication system shown in FIG. 5, when the master station 501 tries to collect vehicle's current state information from each of N units of in-vehicle stations, the master station 501 periodically transmits a polling signal PO to the in-vehicle stations 502-1, 502-2, . . . 502-N to collect the vehicle's current state information from each in-vehicle station as shown in FIG. 6. When each of the n-vehicle stations 502-1, 502-2, . . . 502-N receives the polling signal PO from the master station 501, the in-vehicle stations 502-1, 502-2, . . . 502-N transmit polling response signals S1, S2, . . . Sn including the vehicle's current state information sequentially at a timing previously assigned to each vehicle. The sequence of transmission of the polling response signal after reception of a polling signal by each vehicle is optional, and in the taxi radio communication system, the sequence can be decided, for instance, according to vehicle's number of the vehicles. There is no restriction over the method of deciding the sequence.
For a communication system to be employed in the digital radio communication system as described above, there are the standard specifications based on a narrow band digital communication system such as the digital SCPC (Single Channel Per Carrier) system or the FDMA. In general, the communication system is operated based on these standard specifications. In the standard specifications, a frame format of a radio signal is based on the Japanese standard ARIB STANDARD-T61 (referred to as ARIB STD-T61 below).
FIG. 2 and FIG. 3 illustrate signal frame formats based on the ARIB STD-T61. FIG. 2 illustrates a frame format of a synchronous burst SB. The synchronous burst SB is generally a signal transmitted for establishing synchronicity in communications when a communication channel is set and also when a channel is switched. FIG. 3 illustrates a frame format of a communication channel SC. Formats of the synchronous burst SB and the communication channel SC constitute a frame as a minimum unit of a radio communication signal, and radio communication is performed by repeating the frame. The frame cycle is fixed to 40 ms.
In FIG. 2 and FIG. 3, a portion LP+R indicates a linearizer preamble and bust transient response guard time, a portion P indicates a preamble, a portion RICH indicates a radio information channel, a portion SW indicates a synchronous word, a portion PICH indicates a parameter information channel, a portion G indicates a guard time, and a portion UD indicates a portion not defined yet. Numerals in FIG. 2 and FIG. 3 indicate bit numbers respectively. The abbreviates above are defined in the ARIB STD-T61 described above.
FIG. 4 is a view illustrating an example of a frame transmitted when communication is performed based on the SCPC (Single Channel Per Carrier) system described above. As clearly understood from this figure, to start communication, at first 1 to 3 frames of a synchronous burst SB are transmitted (although 2 frames are shown in FIG. 4), and then voice or non-voice communication is performed via a communication channel SC. Then 2 frames of the communication channel SC comprising an aerial signal are transmitted to notify termination of the communication.
The master station 501 and each of the in-vehicle stations 502-1 to 502-N perform signal transactions by means of the demodulation system of, for instance, π/4 shift QPSK based on the standard specifications. An example of a transmitter at each of the in-vehicle stations 502-1 to 502-N is described below with reference to a block diagram shown in FIG. 7. Also an example of a receiver at the master station 501 is described with reference to a block diagram shown in FIG. 8.
At first, the transmitter shown in FIG. 7 is described. Data for ordinary voice communication or non-voice communication is inputted to an outgoing data input terminal 701. In this step, also data for vehicle's current state information transmitted from each in-vehicle station for polling is inputted to this outgoing data input terminal 701. The vehicle's current state information includes data specific to each vehicle such as positional information about an in-vehicle station (vehicle) on which the transmitter is loaded and information as to whether a passenger is in the vehicle or not.
Data for the vehicle's current state information inputted to the outgoing data input terminal 701 is supplied to the channel encoding section 702. The channel encoding section 702 adds communication information required for communication to the vehicle's current state information inputted to the outgoing data input terminal 701 to generate a frame format for the synchronous burst SB shown in FIG. 2 or for the communication channel SC shown in FIG. 3. The channel encoding section 702 then supplies the frame format as the 384-bit data to a S/P (serial/parallel) converting section 703. The channel encoding section 702 operates under control by a transmission control section comprising a microcomputer or the like not shown, and is switched between a mode for configuring a synchronous burst SB and a mode for configuring a communication channel SC during operation.
When at first a synchronous burst SB is configured to construct an outgoing frame shown in FIG. 4, the channel encoding section 702 arrays data for LP+R, P, RICH, SW, P and G to form the frame structure shown in FIG. 2 with 384-bit data constructed as a whole, and supplies the resulting data as a synchronous burst SB to the S/P (serial/parallel) converting section 703.
When an operating mode is switched to that for constructing a communication channel SC to configure an outgoing frame, the channel encoding section 702 performs encoding to correct errors for the data inputted from the outgoing data input terminal 701 to generate TCH data. The channel encoding section 702 then adds LP+R, P, RICH, SW, and UD data to the TCH data to configure the frame structure as shown in FIG. 3 for forming 384-bit data, and then sends the 384-bit data as a communication channel SC to the S/P (serial/parallel) converting section 703.
Then the S/P (serial/parallel) converting section 703 converts the data inputted from the channel encoding section 702 to parallel data 2 bits by 2 bits with a symbol cycle T and supplies the parallel data to the mapping section 704, where the symbol cycle T is an inverse number of a symbol rate fb, and in the ARIB STD-T61 standard, because the symbol rate fb is equal to 4.8 KHz, the symbol cycle T is 208 μm. Two lines from the S/P (serial/parallel) converting section 703 are connected to the mapping section 704, and 1 bit are inputted through each of the lines to the mapping section 704, namely 2 bits are inputted simultaneously to the mapping section 704.
The mapping section 704 performs mapping in response to the 2-bit data inputted from the S/P (serial/parallel) converting section 703 according to the known I-Q coordinate system. The mapping is described later. As a result of mapping, the inphase component is (I component) is imputed to an upsampler 705-1, while the orthogonal component (Q component) is inputted to an upsampler 705-2. The upsamplers 705-1, 705-2 subjects the inphase component I and the orthogonal component Q of a signal inputted from the mapping section 704 to oversampling, namely, for instance, 16-times oversampling (16 times of oversampling within a symbol cycle), and inputs the resulting components to LPFs (low-pass filter) 706-1, 706-2.
The LPFs 706-1, 706-2 function to restrict a band of signals inputted from the upsamplers 705-1, 705-2 to prevent interference to an adjoining channel. The signals are then converted to analog signals with D/A (digital/analog) converters 707-1, 707-2, and the resulting analog signals are supplied to a transmission high frequency section circuit and a power amplifier 708. The transmission high frequency section circuit and the power amplifier 708 converts the base band signals outputted from the D/A (digital/analog) converters 707-1, 707-2 to signals in a radio frequency band and then supplies the signals, after power amplification, from an outgoing signal output terminal 709 to an antenna not shown in the figure for signal transmission.
FIG. 9 shows an example of configuration of the mapping section 704, and output signals from the S/P converting section 703 (through the two lines described above) shown in FIG. 7 are inputted via bit data input terminals 901-1, 901-2 to a table 902. For the bit data b1 and b0 inputted from the input terminals 901-1, 901-2 to the table 902, the bit data b1 is inputted earlier as compared to the bit data b0 to the S/P (serial/parallel) converting section 703 shown in FIG. 7 (b1 first).
The table 902 is configured with combinations of the input bit data b1, b0 so that each of the values of 1, 3, −1, and −3 can be obtained as an output d. Namely, 1 is obtained as the output data d for the input bit data (b1, b0) of (0,0), 3 for (0,1), −1 for (1,0), and −3 for (1,1). The output d is inputted to an accumulator 903.
The accumulator 903 has an internal memory (a memory in which the content is reset to 0 when power is turned ON). The accumulator 903 adds the content therein to a value d inputted from the table 902, stores a result of addition s again in the memory, and also input the result of addition s to a surplus computing circuit 904. The surplus computing circuit 904 computes a surplus m (=s mod 8) obtained by dividing the output value s from the accumulator 903 by 8, and inputs the surplus m into the table 905.
The table 905 outputs 8 types of mapping value according to a value m inputted from the surplus computing circuit 904, and the inphase component I is inputted to the upsampler 705-1 shown in FIG. 7 via the inphase component output terminal 906-1, while the orthogonal component Q is inputted to the upsampler 705-2 via the orthogonal component output terminal 906-2. Therefore, the inphase component I and orthogonal component Q, which are output values from the table 905, can be developed on an I-Q coordinate plane as shown in FIG. 10.
A receiver at the base station 501 shown in FIG. 8 is described below. An antenna not shown is connected to an incoming signal input terminal 801. A signal transmitted from the transmitter shown in FIG. 7 is received by the antenna, and the incoming signal is inputted to an incoming high frequency wave section circuit 802. The incoming high frequency wave section circuit 802 converts the incoming signal in the radio frequency band to a signal in an intermediate frequency band to supply the same to an A/D converter 803 for digitizing the signal. Then, the digitized signal is supplied to an orthogonal demodulating section 804.
A signal for the inphase component I and a signal for the orthogonal component Q, both of which are transmitted from the receiver, are outputted from the orthogonal demodulating section 804 and are supplied to the LPFs 805-1 and 805-2 respectively. In the LPF 805-1, unnecessary frequency components are removed from the signal for inphase component I, and in the LPF 805-2, unnecessary frequency components are removed from the signal for orthogonal component Q.
The output signals from the LPFs 805-1, 805-2 are supplied to downsamplers 806-1, 806-2 respectively, where only data for one symbol cycle is taken out in the downsamplers and inputted to a demodulating section 807. Timing for taking out the unnecessary frequency components in the downsamplers 806-1, 806-2 is controlled by a timing synchronizing section not shown so that the unnecessary frequency components are correctly taken out according to the symbol timing (in synchronism to a symbol).
In the demodulating section 807, symbol determination is performed according to the inphase component I and orthogonal component Q inputted from the downsamplers 806-1, 806-2, and 2-bit determination data is supplied to a P/S (Parallel/serial) converting section 808 to convert the 2-bit data to serial data, which is inputted to a channel decoding section 809. The channel decoding section 809 separates necessary information and data from the data inputted from the P/S converting section 808, namely decodes a frame structure of a communication channel SC shown in FIG. 3, extracts data from TCH section, decodes the data to obtain incoming data, and outputs the data from an output terminal 810 to supply the data to a data processing section not shown.
It should be noted that FIG. 8 shows a case in which the incoming high frequency wave section circuit 802 operates according to the super heterodyne system. When the incoming high frequency wave section circuit 802 operates according to the direct conversion system, the inphase component I signal and the orthogonal component Q signal are outputted directly from the incoming high frequency wave section circuit 802. In this case, the inphase component I signal and the orthogonal component Q signal outputted from the incoming high frequency wave section circuit 802 are inputted separately via the A/D converter into the LPFs 805-1, 805-2 respectively. Therefore, the orthogonal demodulating section 804 is not necessary.
In the prior art, the system shown in FIG. 4 is applied also to a polling response signal sent from an in-vehicle station to a base station, after a synchronous burst SB is transmitted by 1 to 3 frames, voice communication or non-voice communication is performed via the communication channel SC, and then a communication channel SC comprising an aerial line signal is transmitted by 2 frames to notify an end of communication.
Radio communication between the base station and in-vehicle stations 502-1 to 502-N have been described above. In a radio communication system such as an AVM system for allocating taxies based on the prior art, a current position of an in-vehicle station as a mobile station (a vehicle such as a taxi) is detected with the GPS (Global Positioning System) by and stored in the in-vehicle station itself. Each mobile station returns vehicle's current position information as a response by using a response slot dedicated to each mobile station according to a polling signal cyclically sent from the base station. The base station sequentially performs polling to all vehicles which are mobile stations to grasp current position information about all of the mobile stations. In the system as described above, there is at least one base station which is connected to a management center, and the management center searches an optimal vehicle from the current position information sent from the mobile stations according to a request for allocation of a vehicle from a client and allocates the vehicle (taxi).